Aromatic polycarboxylic acids are commercially important substances as chemical intermediates and have a wide demand as materials of polyesters, polyamides, polyimides and liquid crystalline polymers which are used for fibers, bottles, films and electronic applications. Examples of the aromatic polycarboxylic acids industrially used in the wide range of applications include terephthalic acid, isophthalic acid, phthalic acid, trimellitic acid, pyromellitic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-biphenyldicarboxylic acid, 1,4,5,8-naphthalenetetracarboxylic acid and 3,3′,4,4′-biphenyltetracarboxylic acid.
As the process for producing the aromatic polycarboxylic acid, processes in which an aromatic polyalkylhydrocarbon such as a xylene, a dialkylnaphthalene, a dialkylbiphenyl, a tetraalkylnaphthalene and a tetraalkylbiphenyl is oxidized with molecular oxygen at a high temperature under a high pressure in the presence of a heavy metal such as Co and Mn and a bromine compound in acetic acid as the solvent or oxidized with the air in the presence of nitric acid or chromic acid, are known. The aromatic polycarboxylic acid obtained by the oxidation reaction contains impurities such as monocarboxylic acids and aldehydes which are intermediate products of the oxidation reaction, addition products of bromine and metal components which are derived from the catalyst, and coloring substances having unknown structures.
When the aromatic polycarboxylic acid containing impurities is used as the material in the polymerization with a diol or a diamine, the obtained resin exhibits inferior physical and mechanical properties such as inferior heat resistance, mechanical strength and dimensional stability. Therefore, the aromatic polycarboxylic acid containing impurities cannot be used as the materials for polyesters, polyamides and polyimides. In general, crude aromatic polycarboxylic acids obtained by oxidation are colored yellow or black and cannot be used for applications requiring transparency such as bottles and films without further treatments. Moreover, since particles of crude aromatic polycarboxylic acids have, in general, small diameters, handling is not easy, and problems tend to arise during polymerization. Therefore, the process for industrially advantageously producing a high purity aromatic polycarboxylic acids having improved hue and particle diameter has been studied for a long time.
For example, in Japanese Patent Application Laid-Open No. Heisei 7(1995)-149690, a process in which a p-phenylene compound is oxidized in the liquid phase, the product is catalytically hydrogenated in water at a high temperature, the obtained slurry of terephthalic acid is introduced into a column for substituting solvents and brought into contact with an upward stream of water at a high temperature which is introduced at a lower portion of the column for substituting solvents, crystals of terephthalic acid are taken out of the bottom portion of the column as a slurry together with water at a high temperature, and crystal of a high purity terephthalic acid are separated, is described as the process for producing a high purity terephthalic acid.
In WO 02/088066, a process in which a crude aromatic polycarboxylic acid is formed into a slurry in an aqueous medium and brought into contact with a metal catalyst in the absence of oxygen while contamination of crystals with the catalyst components is prevented, is described.
In Japanese Patent Application Laid-Open No. Heisei 9(1997)-151162, a process in which a crude naphthalenedicarboxylic acid is dissolved into water at a high temperature under a high pressure, treated by reduction in the presence of a hydrogenation catalyst and then washed with a lower aliphatic carboxylic acid, is described. In U.S. Patent Application Laid-Open No. 2002/0002303, a process in which a crude naphthalenedicarboxylic acid is brought into contact with a metal of the Group VIII in the presence of hydrogen, is described.
In general, the purification of an organic compound is conducted in accordance with the process of distillation, crystallization or adsorption or a combination of these processes. However, since an aromatic polycarboxylic acid has a temperature of self-decomposition lower than the boiling point, the purification by distillation is substantially impossible. Since the solubility of an aromatic polycarboxylic acid in industrially widely used solvents is small, it is difficult that the easy purification by crystallization is conducted. In particular, naphthalenepolycarboxylic acids and biphenylpolycarboxylic acids are not easily dissolved in various solvents, and no industrially advantageous processes for producing high purity naphthalenepolycarboxylic acids and high purity biphenylpolycarboxylic acids have been established.
In the process in which an aromatic polycarboxylic acid is purified by completely dissolving the acid, a great amount of steam is necessary, and the cost of the utility is excessively great. In the processes for purifying an aromatic polycarboxylic acids described above, the acid is purified using a slurry having an increased concentration to decrease the consumption of energy. However, it is difficult that crystals of a high purity aromatic polycarboxylic acid having excellent hue and particle diameter are obtained in accordance with these processes.
The present invention has an object of providing a process for industrially advantageously producing an aromatic polycarboxylic acid having excellent hue and particle diameter while the construction of the process is simplified and the consumption of energy is decreased.